The art of petroleum refining and specifically the area of motor gasoline manufacture seeks to maximize the market value of a produced crude oil by weighing market demands against capital equipment and energy costs to define an optimum product distribution. The advent of higher performance automotive engine designs has shifted gasoline demand in recent years, notably increasing both the volumetric demand for premium gasoline as well as for the octane level required. Gasoline yield and octane rating are in fact so commonly considered together that the term "octane-barrel" has been defined by the industry as the multiplicative product of the gasoline octane rating and the produced volume in units of barrels.
Previous octane-enhancing processes generally imposed a liquid product penalty in that a portion of the liquid feedstock was converted to light C.sub.4 - gas rather than to liquid gasoline. The inverse relationship between gasoline volumetric yield and octane rating posed a particularly perplexing problem to the refining industry in view of changing market demands.
For example, a typical catalytic reforming process upgrades paraffinic naphtha to high octane reformate over a metallic catalyst in the presence of hydrogen. Increasing severity (e.g., reactor temperature) produces a higher octane liquid product but also shifts selectivity away from the liquid product toward less valuable C.sub.4 - light aliphatic gases. Thus the incremental value of increasing reformate octane is mitigated to a certain degree by lost gasoline volume.
Gasoline additives, e.g., tetraethyl lead, present another option for meeting octane barrel requirements. While various refinery streams respond differently to such additives, lead additives improve octane in almost all refinery gasoline streams, and certain streams such as alkylate gasoline from a sulfuric or hydrofluoric acid alkylation unit show marked improvements in motor (MON) and research (RON) octane numbers. The widespread use of these additives is however, being phased out to decrease automotive exhaust emissions.
Research efforts have more recently focused on upgrading gasoline by blending methyl, propyl or isopropyl ethers of tertiary butyl ether with gasoline range hydrocarbons, and further on producing these ethers at a commercially competitive cost. Examples of such processes are taught in U.S. Pat. Nos. 4,664,675 and 4,647,703 to Torck et al. These processes feed an olefinic gasoline to an etherification zone where the gasoline in reacted with methanol to obtain an effluent containing methyl tertiary amyl-ether. The unreacted methanol is extracted with water and the aqueous extract is fractionated to recycle unreacted methanol. The operating costs associated with the extract fractionation column impose an economic burden which can reasonably be expected to worsen with rising energy costs.
U.S. Pat. No. 3,904,384 to Kemp teaches a process for producing ether-rich gasoline from a single source of C4 hydrocarbons by hydrating isobutane with propylene to obtain isopropyl tertiary butyl ether which is then blended with a gasoline stream.
U.S. Pat. No. 4,393,250 to Gottlieb et al. discloses a process for etherifying isobutylene by first hydrating propylene to isopropyl alcohol and then etherifying the isobutylene with the produced isopropyl alcohol.
The ability of lower alkyl ethers to enhance octane has drawn attention primarily to the use of methanol to etherify isobutylene to form MTBE, or to etherify isopentane (isoamylene) to yield tertiary amyl-ether (TAME). Methanol is both relatively inexpensive and readily available. Further, methanol is known to etherify isoalkenes more readily than secondary or tertiary olefins. For example, U.S. Pat. No. 4,544,776 to Osterburg et al. cites methanol as a preferred alcohol for the etherification of C.sub.4 -C.sub.7 olefins. The specific olefinic gasoline feedstocks useful in the present invention are relatively undesirable as motor gasolines. To upgrade their characteristically low octane, such streams have been proposed as feedstocks for catalytic aromatization processes such as the Mobil M-2 Forming process. While aromatization clearly achieves the objective of increased octane rating, the process decreases product volume.
Clearly then it would be desirable to provide an energy efficient process for upgrading the market value of C.sub.3 -C.sub.8 olefinic gasolines without producing substantial quantities of less valuable light aliphatic gases.